In recent years, automation in various types of tests such as clinical laboratory tests and reduction in the assay time thereof have been tried. As a method of these tests, an assay utilizing an immune reaction is widely used for measurement of a substance in a biological sample. Examples of the immunoassay include many methods such as RIA, EIA, immunonephelometry, latex agglutination, colloidal gold agglutination, and immunochromatography. Among such methods, the latex agglutination and the colloidal gold agglutination are capable of measurement in a homogeneous system in which the separation or washing operation of a reaction mixture is not required, and therefore suitable for automation of determination and short-time assay. In particular, colloidal gold particles have a size of 5 nm to 100 nm, which is smaller than the size of latex particles, so that colloidal gold particles can be used in an assay of a tracer substance (Japanese Laid-Open Patent Publication Nos. 2005-283250 and 2004-325192).
In these assays, the principal reaction component is the microparticle such as latex particle or colloidal gold particle having a substance that specifically react (for example, bind) with an analyte bound thereto. The specific binding substance such as antibody present on the microparticle can bind with an analyte, thereby inducing the agglutination of microparticles. This agglutination occurs in a manner dependent on the amount of analyte, and thus a mechanical measurement of this phenomenon enables the mass of analyte to be calculated.
However, such an analyte of low molecular weight as medicaments or chemical substances has a small number of sites for binding to its specific binding substance, so that agglutination reaction is unlikely to be induced, making it difficult to construct a homogeneous measurement system using an agglutination reaction. Therefore, it has been necessary to especially add a competitor carrying a hapten bound thereto to a reaction system (see, for example, Japanese Laid-Open Patent Publication Nos. 2004-325192 and 2006-38594).